The mature central nervous system exhibits the capacity to alter cellular interactions as a function of the activity of specific neuronal circuits. This capacity is believed to underlie learning and memory storage, age-related memory loss, tolerance to and dependence on drugs of abuse, recovery from brain injury, epilepsy as well as aspects of postnatal development of the brain (Schatz, C., Neuron, 5:745, 1990). Currently, the role of activity-dependent synaptic plasticity is best understood in the context of learning and memory. Cellular mechanisms underlying activity-dependent plasticity are known to be initiated by rapid, transmitter-induced changes in membrane conductance properties and activation of intracellular signaling pathways (Bliss and Collingridge, Nature, 361:31, 1993). Several lines of evidence also indicate a role for rapid synthesis of mRNA and protein in long-term neuroplasticity. For example, classical studies of learning and memory demonstrate a requirement for protein synthesis in a long-term, but not short-term memory (Flexner, et al., Science, 141:57, 1963; Agranoff, B., Basic Neurochemistry, 3rd Edition, 1981; Davis and Squire, Physiol. Bull., 96:518, 1984), and long-term enhancement of synaptic connectivity, studied in cultured invertebrate neurons (Montarolo, et al., Science, 234:1249, 1986; Bailey, et al., Neuron, 9:749, 1992) or in the rodent hippocampus (Frey, et al., Science, 260:1661, 1993; Nguyen, et al., Science, 265:1004, 1194), is blocked by inhibitors of either RNA or protein synthesis. Importantly, inhibitors of macromolecular synthesis are most effective when administered during a brief time window surrounding the conditioning stimulus indicating a special requirement for molecules that are rapidly induced (Goelet, et al., Nature, 322:419, 1986).
Immediate early genes (IEGs) are rapidly induced in neurons by neurotransmitter stimulation and synaptic activity and are hypothesized to be part of the macromolecular response required for long-term plasticity (Goelet, et al., supra; Sheng and Greenberg, Neuron, 4:477, 1990; Silva and Giese, Neurobiology, 4:413, 1994). To identify cellular mechanisms that may contribute to long-term plasticity in the vertebrate brain, differential cloning techniques have been used to identify genes that are rapidly induced by depolarizing stimuli (Nedivi, et al., Nature, 363:713, 1993; Qian, et al., Nature, 361:453, 1993; Yamagata, et al., Neuron, 11:371, 1993; Yamagata, et al., Learning and Memory 1:140, 1994; Yamagata, et al., Journal of Biological Chemistry, 269:16333, 1994; Andreasson and Worley, Neuroscience, 69:781, 1995; Lyford, et al., Neuron, 14:433, 1995). In contrast to the earlier focus on transcription factors, many of the newly characterized IEGs represent molecules that can directly modify the function of cells and include growth factors (Nedivi, et al., supra; Andreasson and Worley, supra), secreted enzymes that can modify the extracellular matrix, such as tissue plasminogen activator (Qian, et al., supra), enzymes involved in intracellular signaling, such as prostaglandin synthase (Yamagata, et al., supra), and a novel homolog of H-Ras, termed Rheb (Yamagata, et al., supra), as well as a novel cytoskeleton-associated protein, termed Arc (Lyford, et al., supra). The remarkable functional diversity of this set of rapid response genes is representative of the repertoire of cellular mechanisms that are likely to contribute to activity-dependent neuronal plasticity.
The identification of molecules regulating the aggregation of neurotransmitter receptors at synapses is central to understanding the mechanisms of neural development, synaptic plasticity and learning. The most well characterized model for the synaptic aggregation of ionotropic receptors is the neuromuscular junction. Early work showed that contact between the axon of a motor neuron and the surface of a myotube rapidly triggers the accumulation of preexisting surface acetylcholine receptors (Anderson and Cohen, J. Physiol. 268:757–773, 1977; Frank and Fischbach, J. Cell. Biol. 83:143–158, 1979). Subsequent work has shown that agrin, a complex glycoprotein secreted by the presynaptic terminal, activates a postsynaptic signal transduction cascade (reviewed by Colledge and Froehner, Curr. Opin. Neurobiol. 8:357–63, 1998), that leads to receptor clustering by the membrane associated protein rapsyn.
In the central nervous system, ionotropic glutamate receptors are the major excitatory neurotransmitter receptors and are divided into three broad classes, termed AMPA, NMDA and kainate type receptors, on the basis of molecular and pharmacological criteria (Hollmann, M., and Heinemann, S., Ann. Rev. Neurosci. 17:31–108, 1994). The predominant charge carrier during routine fast excitatory synaptic transmission is the AMPA type receptor, while NMDA receptors contribute a significant calcium current, which is thought to modulate signal transduction pathways. Functional AMPA receptors are multimeric complexes of the homologous subunits GluR1–4 (Rosenmund et al., Science 280:1596–9, 1998; Mano and Teichberg, Neuroreport 9: 327–31 1998) which share about 60% to 70% homology at the amino acid level (Keinanen et al., Science 249:556–60, 1990). A variety of studies have shown that glutamate receptors are highly concentrated in neurons at excitatory synapses on dendritic spines and shafts.
Significant advances in the identification of molecules involved in excitatory synapse formation have recently occurred using genetic and biochemical techniques. A family of cytoplasmic proteins containing protein—protein interaction motifs, called PDZ domains, have been implicated in the clustering of both NMDA and AMPA receptors at synapses (O'Brien et al., Curr. Opin. Neurobiol. 8:364–9 1998). These PDZ domain containing proteins are thought to intracellularly cross link receptors and couple them to the cytoskeleton. The PSD-95 family of proteins contain three PDZ domains, which directly interact with the C-termini of NMDA receptor subunits and may be important in NMDA receptor clustering (Komau et al., Science 22:1737–40, 1995). Similarly, the neuronal proteins GRIP (Dong et al., Nature 386:279–84, 1997), ABP (Srivastava et al., Neuron 21:581–91, 1998), and Pick1 (Xia et al., Neuron 22:179–187, 1998), each of which contains one or more PDZ domains, interact with the C-terminus of AMPA receptors and may be important in receptor targeting (Dong et al., supra). The extracellular factors that facilitate the formation of excitatory synapses in the central nervous system have not been identified.
An additional level of complexity in the formation of central excitatory synapses stems from the fact that two populations of neurons exist (termed spiny and aspiny) which receive excitatory input in mutually exclusive patterns (Sloper and Powell, 1979; Harris and Kater, 1994). Spiny neurons, such as hippocampal pyramidal neurons receive more than 90% of their excitatory input onto dendritic spines, while shaft synapses on these neurons are largely inhibitory. Aspiny neurons such as hippocampal interneurons and most spinal neurons receive both excitatory and inhibitory synapses on their dendritic shafts. Emerging evidence indicates that excitatory synapses on spines and shafts have different structural and functional properties which may imply different molecular mechanisms in their formation and maintenance (O'Brien et al., J. Neurosci. 17:7339–50, 1997; Rao et al., J. Neurosci. 18:1217–29, 1998, amongst others).